Current Concepts in Acceleration Physiology
by
Wg Cdr A Krishnamurthy
Cl Spl (Av Med)
IAM, IAF

Panelists:
Wg Cdr P Kharbanda
Wg Cdr S Modak
Wg Cdr P K Tyagi
Maj A Chawla

INTRODUCTION
"It is not the resistance of material which limits the aerobatic performance of the artificial bird, but the physiologic resistance of man, who is the brain of the artificial bird"(Bleriot, 1922). Man is the weakest link in this man-machine interface and has been so for the past 97 years-ever since Orville and Wilbur Wright flew at Kitty Hawk into fame. The first problem in acceleration physiology noted was in 1918 during the Schneider Cup air race, where a tight turn before the return leg meant victory. Initially described as "faintness in the air" and later came to be known as G-LOC. Then during the 30s' and 40s', the Germans experimented with live subjects and determined various tolerance levels of the human body. The aircraft industry was far ahead of the human tolerance levels and in the 60s' and 70s' produced aircraft whose G- loading was in the range of 6-9G. It soon improved to 12G with onset rates upto 6G/s and since then we have witnessed various aerodynamic changes in the aircraft that would keep it secret from the enemy or perform in such a manner to come out on top, literally. The 21st century fighter aircraft and their capability for rapid onset, high-sustained acceleration has lent new urgency to Bleriot's observation. Future weapon systems may well demand more of man. As aircraft become extremely maneuverable, the human body has to respond to the rapidly changing acceleration stress in different axis. These aircraft, termed the ASF/HP and the super-maneuverable/super agile(SM/SA) aircraft are capable of attaining 12G with onset rates upwards of 15 G/s. So, the human being will experience physiological problems due to the rapid change in direction of acceleration forces from one axis to another. Protective clothing by means of the AGS (Anti -'G' Suit) has come a full circle from Frank's water filled suit to the 'Libelle' suit, currently undergoing field evaluation; from the cover all -type of AGS to the ECAGS (extended coverage) and ATAGS (Advanced technology anti-G suit). Advances in anti-G valve technology have progressed from mechanical anti-G valve to microprocessor controlled high flow and high flow ready pressure valves. Numerous types of AGSM are available of the shelf, but which is the one to choose to get maximum protection? Which AGSM gives more protection, with less fatigue and how easy are they to learn and implement? What is the role envisaged for PBG in the modern aircraft and how effective would it be? The latest system to deliver PBG i.e. COMBAT EDGE (Combined Advanced Technology Enhanced Design G Ensemble), is expected to give a protection of 12G. The role of training on the ground in a Human Centrifuge still remains the best option. The current and future generation Centrifuge hold great promise, as they can simulate G profiles of the latest aircrafts, with a onset/offset rates of 12-15G/s, with active control gondolas and full flight simulation. Therefore, is training and education on ground a must for aircrew of the SM/SA aircraft? The future holds more questions than answers. However, research will continue towards achieving the goal of relaxed 9Gz and straining 12Gz tolerance, which is not far away and will not remain elusive in the years to come.

The current concepts in acceleration physiology would therefore, be discussed under the following:

SUPERMANOEUVERABILITY
From the days of piston e0ngine aircraft we have come to the era of jets and "super jets" Aircraft technology has advanced to give us the modern high performance aircraft, which not only pulls high G but also are highly maneuverable. In future the super-manoeuverable or Superagile aircraft with vectored thrust control and with variable wing geometry will be the order of the day. However, the human being still remains the same, as he/she was earlier, with the same physiological limitations. There are various aspects to the problem posed by the progress in aircraft technology.

Aircraft Characteristics
Supermanoeuverability is defined as the capability to execute maneuvers with controlled sideslip at angles of attack well beyond those for maximum lift. Dr. WB Herbst of West Germany first coined this term in 1980. It is also termed as Post stall maneuvering and is one of the many important capabilities included in the category of supermanoeuverability. Other capabilities which supermanoeuverability includes are increased usable lift, dynamic lift overshoot, thrust vectoring, and unsteady aerodynamic effects used in synergistic fashion as a means of obtaining greatly enhanced maneuverability.

These capabilities will include multi-axis accelerations, and unprecedented angular velocities and accelerations around the three principal axes of the aircraft. The new issues, which arise in this environment, are pilot capability involving biodynamics, acceleration tolerance, performance and spatial disorientation.

Angular excursions of current aircraft about their principle axes are limited ordinarily to roll (360 degree/sec) and pitch (max 20 degree angle of attack at a rate of 15 deg/sec), with very limited yaw, used only in sideslip manoeuvres. There is shedding of nose vortices at high angles of attack and this causes the aircraft to yaw to the left or to the right. The combination of these two effects, results in stalling of the nose-high airfoil, followed by departure into a spin; usually a flat spin in modern fighter aircraft.

By contrast, SM/SA (superagility) are capable of significant excursions and accelerations in the X and Y directions as well as high sustained Gz load vectors. These capabilities depend on the incorporation of new canard and wing geometrics and the use of one-or-two-dimensional vectored thrust. They exploit maneuver capabilities obtained by flying at low airspeeds and at high angles of attack, a region generally referred to as post-stall regime. Thrust vectoring contributes to very rapid rates of pitch and yaw motions. This Post Stall Thrust (PST) helps to rapidly point the nose of the aircraft in virtually any desired direction.

The aircraft having such capabilities have certain inherent disadvantages and problems.

Once in the post stall regime, the pilot becomes vulnerable to attack by the opponent's wingman, as the aircraft will be in a very low speed state, at the end of the supermanoeuvre. The break turn defensive maneuver could be valuable since it would cause an opponent to overshoot and enter into a gun or missile shot envelope. However, no country has missiles which can be launched at such high angles of attack.

PST opens up a slew of problems for which training schedules must be prepared. For acceleration tolerance, SM is both good news and bad news. While PST maneuvers reduce Gz accelerations, Gx and Gy now increase. Though new and stronger seat restraints can be designed, the problem with neck injuries may actually increase, since the head of a fighter pilot cannot be restrained without inhibiting his performance.

From the bio-dynamics point of view, the physiological tolerances of high Gx and Gy accelerations need to be researched further. SD could be more serious as the body's physiological orientation mechanisms are exposed to rapid and unusual moves required by PST. Research in this area is virtually non-existent. With such unconventional maneuvers happening very quickly, can the human orientation mechanisms maintain a proper orientation reference while under the stress of close air combat?

Probably the greatest problem in this area will be situational awareness. PST maneuvers require a lot of concentration and attention to the target because of the complexity of the maneuvers. In a fury of air combat, the workload and level of attention necessary may be something that pilots cannot handle or afford. It is conceivable that target fixation, task saturation and channelised attention could become greater, than they are now.

Though the consensus is that it will be a welcome addition to the fighter pilot's armamentarium, but not if it requires elimination of other significant fighter performance capabilities

Physiological Effects of acceleration in different axes and effects of multi-axis accelerations:
Man has evolved in the gravitational environment of the earth. He has thus in a sense adapted to terrestrial gravity. The homeostatic mechanisms that respond to the stimulus of gravity are put to test whenever man is exposed to increased G forces. The G-induced homeostasis has a remarkable physiological reserve thus allowing man to withstand exposure to several multiples of G. The various physiological systems of the body respond to the acceleration forces depending on the magnitude and direction of these forces. The following are the known effects of various acceleration forces on different physiological systems of the body.

+Gz acceleration : +Gz acceleration initiates responses on the CVS that are similar to those that are genetically designed to respond to natural stresses such as orthostasis, exercise, hypoxia and hemorrhage. These changes are initiated by a decrease in the arterial blood pressure leading to a baroreceptor mediated increase in sympathetic and reduced parasympathetic outflow to the heart and peripheral vasculature. There is an increase in cardiac contractility, heart rate and total peripheral resistance. The increased heart rate helps maintain the cardiac output in spite of a reduced stroke volume. The limiting factor for +Gz tolerance is the ability to maintain arterial blood pressure at the level of the head.

During exposure to HSG there is a significant reduction in absolute plasmavolume, as plasma leaves the vascular compartment due to increased hydrostatic pressure. One minute at 6G results in a 11% reduction in plasma volume. The volume returns to normal in about 30 minutes at 1G.

The heart rate during sustained G increases rectilinearly till 7G, where it is about 170 bpm. Ventricular ectopics may occur due to the increased sympathetic activity. Certain individuals show a paradoxical bradycardia during +Gz stress. This could be the result of baroreceptor stimulation due to AGSM or the unmasking of a protective reflex seen during hemorrhage, to limit blood loss.

The respiratory effects on exposure to sustained +Gz acceleration leads to alterations in the ventilation and perfusion of the lungs, thus changing the V/Q ratio. The V/Q inequality normally present at 1G is exaggerated. Alveoli at the lung bases may collapse (absorption or compression atelectasis). This produces a shunt-like situation. At the apical areas of the lungs it is the perfusion that reduces. These changes cause a reduction in the PaO2; PaCO2 remains normal.

Changes in cerebral blood flow occur under Gz. In one particular study the middle cerebral artery flow was found to reduce by 49% during 4G. The reduction of cerebral blood flow is of a less magnitude compared to the fall in head level arterial pressures. This is so since the skull is a bony structure with a constant volume and increasing G causes a reduction in CSF fluid pressure. This reduces the extravascular compression of cerebral vessels. The fall in jugular venous pressures at the same time produces a siphon effect keeping the a-v pressure differential at about 50mmHg.

-Gz Acceleration : The effects produced on the CVS are essentially the opposite of those seen during +Gz acceleration. The increased headward blood flow results in a baroreceptor-mediated bradycardia. Exposure to -Gz alternating with +Gz can cause wide fluctuations of the heart rate of >100 bpm. There are reports of heart rate fluctuations from 175bpm to 40 bpm during such exposures. The fear of cerebral capillary rupture due to -Gz exposure is not high since the concomitant increase in CSF pressure affords adequate protection. There , however, can be discomfort as a result of head/face soft tissue oedema which can limit human tolerance. Thus, human tolerance to -Gz can be quite high if one can bear the discomfort.

±Gx Acceleration : The effects of Gx acceleration on the CVS are insignificant since the height of blood column ceases to be a significant factor. Positive Gx accelerations have, on the contrary, been reported to increase cardiac output. A study reported an 11% increase in cardiac output and a 35% increase in heart rate during +5Gx acceleration. Mean arterial pressure increased by 17%. Another study found a 25% increase in systolic blood pressure at +8Gx. Negative Gx acceleration causes almost similar changes. There is a likelihood of cardiac rhythm disturbances during +Gx acceleration.

Unlike the effects on the CVS, respiratory effects differ in +Gx and -Gx accelerations. The vital capacity is seen to reduce by almost 75% at +6Gx, while the reduction is only 15% during -6Gx. Alveolar ventilation reduces by 50% at +8Gx but increases to almost 150% at -8Gx. These differences between + and - Gx accelerations occur because of restriction of lung movement by the shape of the diaphragm and the spine during +Gx. This is not so during -Gx. Positive Gx can cause greater reduction in arterial partial pressure of oxygen than -Gx. Absorption atelectasis is seen to occur at +5.6 to +6.4 Gx while breathing 100% oxygen. The limits of human tolerance to ± Gx acceleration is about 15 Gx. Prolonged exposure results in increased difficulty in breathing and soft tissue oedema of the throat region.

Gy Acceleration : The effects of transverse acceleration on the cardiovascular system are seen to be greater than that of Gx acceleration. A 30% reduction in arterial pressure was seen at +4Gy exposure for 1 minute. The effects on the respiratory system are not significant.

Multi-axis Accelerations : Modern, thrust-vectored aircraft have the capability of developing multi-axis accelerations, especially during the performance of supermanoeuvres. These agile aircraft are capable of unconventional flight. The multi-axis environment was studied on a gimbaled centrifuge. Effects of sustained Gx & Gy on Gz tolerance is not known. Research is on. As an end point subjects have been trained to recognize visual symptoms. Either +/-2Gy & +/-4Gx were used alternately for seated relaxed unprotected subjects followed by +Gz at 0.1G/sec starting from +1.4G till they lost vision. Heart rate, % cerebral O2 saturation, and % change in cerebral blood volume were collected during each exposure. The results are:

• Moderate -Gx acceleration significantly reduced +Gz tolerance by 0.25G.



• Moderate lateral acceleration significantly increased +Gz tolerance by 0.5G.



• -Gx significantly reduced cerebral blood volume as well compared to +Gz alone.



• +/- Gy acceleration reduces tracking task performance



• +Gz tolerance reduces when preceded by -Gz

Therefore, certain conclusions can be drawn from the current knowledge available. These are :

• Multi-axis sustained accelerations; such as those experienced when flying thrust vectored aircraft manoeuvre, can either enhance or reduce G-tolerance of the pilot depending upon the direction and duration of the exposure.



• Lateral acceleration in conjunction with vertical acceleration can enhance G- tolerance.



• Longitudinal accelerations in addition to vertical acceleration can reduce G- tolerance.



• - Gz acceleration reduces +Gz tolerance.

Despite the knowledge of these effects, it is still not very clearly known how an individual will respond to sudden changes in the direction of the acceleration vector. The effects of alternating exposure to + and - Gz acceleration on cerebral blood flow are well known. There are no published reports available on the effects of alternating exposure to ± Gx and ±Gy. There is also no published data available on the effects of acceleration in a particular axis affecting the physiological responses to acceleration in another axis. These are problem areas as far as human responses to super maneuverable aircrafts are concerned. Detailed studies are required to assess such changes and effects. These of course will require the availability of facilities to simulate acceleration in all 3 axes as well as the availability of sophisticated and accurate physiological monitoring and recording systems.

PROTECTIVE MEASURES
Several protective strategies to increase G tolerance and to prevent G-LOC have been under development and in some cases already put into operational use. These strategies include new G valves and G suits, the use of positive pressure breathing both assisted (with counterpressure) and unassisted, altering the seat back angle in the aircraft physical conditioning and centrifuge training.

AGV( Anti-G Valve)
Performance requirements for advanced Anti-G Valves for a modern high performance fighter should raise the suit pressure as quickly as possible; preferably in less than one second and should provide a rapid acting mechanism to raise the suit pressure to a tolerable limit to inhibit early venous pooling.

Recent research in Armstrong Aerospace medical research laboratory has shown the limitations of traditional equipment through the use of two-dimensional echocardiography. A study was done to see the effects of more rapid anti-G suit inflation on cardiac volumes using 2-d echo. Subjects were exposed to +4Gz for 30seconds under three conditions:

• Wearing an uninflated antiG suit
• Wearing an anti-G suit inflated by a High Flow Only (HFO) valve
• Wearing an anti-G suit inflated by the valve in current use in the fleet.

The HFO valve inflates the anti-G suit some 33% more rapidly than the standard valve. Using the HFO valve, the initial decrease in EDV, SV, and CO was less than in other two conditions. So, a rapid acting valve augments blood return initially and is thus more effective in mitigating the effects of a rapid onset acceleration stress. It has been seen that a conventional valve with a conventional suit is incapable of providing protection over an extended period because it appears to act as a venous occlusion cuff.

The advanced Anti-G valves currently in development are given below :

Servo-controlled Anti-G Valve:The US Navy first sponsored the development of this valve in 1995. It is also known as servo-controlled rapid acting anti-G valve (SACG). The anti-G suit-filling schedule (suit pressure versus +Gz), is defined by the electronic controller (may be analog or digital in design) making use of the accelerometer inputs, and the pressure feedback loop. The feedback pressure is sensed at the valve outlet. A time-delay circuit is incorporated in the feedback transducer line to compensate for the pressure-time histories between the valve outlet and the suit inlet. This time delay, thus, forces the electronic controller to drive the valve wide open briefly in order to conform to the specified G vs. pressure schedule. Specified schedule is same as traditionally used for anti-G valves i.e. linear acceleration and does not address the issue of acceleration onset rate. Incorporating rate sensitivity and a modified time delay algorithm could improve the performance of this valve.

Bang-Bang Servo Anti-G Valve :: The SCAG valve just discussed is a proportional controller. The output of the valve is proportional to some control signal. There is another type of controller, such as the thermostat of air conditioner, which is called a 'bang-bang' controller. This type is either on or off, depending upon the presence or absence of a control signal. It was developed at the Harry G Armstrong Aerospace Medical Research Laboratory. The design criteria were: rapid inflation to 5psig, and subsequent inflation to the highest permissible pressure in the early portion of a high onset rate episode. Other criteria was least cost, least risk and least aircraft interface approach. The objective of the effort was to achieve a quick and effective retrofit for the F-16. To summarize the features :

• It is a valve that is sensitive to both acceleration rate and magnitude.



• If the sensed acceleration is in excess of +1.5Gz and if the onset rate exceeds
  approximately 1.5G/sec, a solenoid is used to drive an HFO valve to the full open
  position for a period of 2.5 second. This length of time is sufficient to assure
  that any size anti-G suit will be inflated to the maximum pressure permitted by
  the regulating portion of the valve.

Adapted 1533 Interfaced Servo valve : This valve has the following features;

• The dedicated microprocessor associated with this valve design responds to flight control inputs in order to achieve the most rapid response possible.
• By virtue of the associated software, the valve will know the current and antecedent acceleration history of the aircraft.

AGS: (Anti-G Suit)
Franks in Canada developed the first workable anti-G suit during World War II. Simultaneously Cotton in Australia developed the first air filled suit. Franks suit was not acceptable operationally because it was water filled, but it laid the groundwork for what was to follow. The current USAF anti-G suit is the CSU 3-B/P, which has calf, thigh, and abdominal air bladders that can be inflated to a maximum of 10.0 PSI. This suit must be individually fitted and provides about 1 +Gz of protection. Most of the protection seems to be provided by the abdominal pressure bladder or the combination of all the bladders as inflation of the leg bladders alone only provides 0.2 G increase in +Gz tolerance.

The requirements of a modern AGS should fulfill the following criteria :

• Inflation rate: 1.5 PSI/G starting at 2G to a maximum of 10.5 PSI. The suit does not inflate below 2G and therefore buffeting of the aircraft and low levels of G in moderate turns will not cause the AGS to inflate.



• Maximum lag period of suit inflation: within 1 second after obtaining the maximum G level.



• Too rapid inflation will cause discomfort. Diaphragm stretching probably causes this.



• Pre G exposure inflation will result in increase in the Pa above 120 mmHg. This will cause activation of the baroreceptor responses, resulting in vasodilatation and unwanted drop in the arterial BP.

Towards this end, the recent advances in AGS technology in the different Air Forces of the world are given below. There is some overlap in the characteristics of these Anti-G suits, as is brought out in the succeeding paragraphs.

Extended Coverage Bladder (ECB) G Suits have the following features :

• Covers 85% of body surface area below umbilicus



• 5 bladders completely encircled legs & lower trunk



• Standard suit covers 30% of body surface area



• Cases of severe abdominal pain and foot pain are reported with the standard suit.



• Inflation limiters are used for abdominal and thigh bladders.



• Foot bladders have also been added.



• ECB (unmodified) provides almost +1Gz protection more than the standard suit. ECB (modified) provided similar protection. Addition of foot bladders provided another +0.5 Gz protection.



• Other models of the extended bladder suits are :
  
  TLSS (Tactical life support system) AGS suit
  
  Navy EAGLE (Enhanced Anti-G lower ensemble) anti-G suit



• The concept of the extended coverage is linked to the utilization of PBG. Though this method increases the G-time tolerance it also causes the chest pressure to increase above normal and therefore, potentially reduces the venous return. For this reason the ECAGS was developed to enable better venous return.

Full Coverage Anti-G trousers :

• It is a part of the Enhanced G protection system (UK), which comprises of Full coverage Anti-G trousers (FAGT), PBG and Chest counter pressure. The Swedish developed a similar Enhanced G protection system for the Grippen aircraft.



• Bladder coverage extends from the umbilicus down to the toes and cover 93% of the lower body.



• Areas not subject to counter pressure are the crotch, saddle area and small portion of the ankle around the malleoli.



• The FAGT is manufactured from butyl-nylon-butyl composite materials, stitched and taped on the seams



• In the anterior abdominal area, restraint tabs were inserted, to reduce the tendency of this part of the bladder to become a spherical shape upon inflation.



• Provides mean relaxed G-tolerance of 8.3G. Therefore, only a moderate degree of muscle tensing/AGSM is required to give the desired level of protection. The level of fatigue developed is also less.



• There is difficulty in donning and doffing, decrease in the mobility, increased heat stress and difficulty in putting on shoes after wearing the FAGT.

Advanced Technology AGS (ATAGS) in the F-16 :

• Development started in the Armstrong Laboratory in the 1970's and has evolved from the full coverage suit to what it is now called ATAGS. The ATAGS is an extended coverage anti-G suit with full pneumatic bladder coverage of the legs from the hip to the ankle and an abdominal bladder that is lower in height, by approximately 2 inch, at the top edge, when compared to the CSU-13B/P anti- G suit, and thus the ATAGS abdominal bladder is small in volume.



• ATAGS is an enhanced coverage anti-G garment, which provides 90% below the waist coverage. It is the first significant improvement in anti-G garment since W.W.II. ATAGS is a full lower body coverage trouser, which may be worn with or without foot pressure socks. Recently, the ATAGS concept was shown to provide improved acceleration tolerance as measured by endurance type acceleration profiles. It works best in combination with PPB - yielding a 400% improvement in G tolerance. Alone, ATAGS has a 60% improvement over the standard G- suit. ATAGS is currently fielded, but its final development and production are anticipated in the future.

Libelle AGS :

• Designed by LSS AG (Switzerland), this suit uses fluid filled channels traversing the arms, torso and legs to tension its snag fitting fabric in simulation of water immersion. Andreas Reinhard, a 45-year-old former pilot in the Swiss Air force, spent 13 years and millions of dollars in venture capital developing a G-suit that works in a completely different way. Libelle, the German word for dragonfly, because it is based on the same principles that protect a dragonfly's innards from the 30- G accelerations the insect generates in flight. When the blood rushes to one side of the body, so does the liquid, providing a counter pressure. It contains one third of a gallon of fluid (water) in sealed tubes that run from neck to ankle. With increasing +Gz, the water in the Libelle suit rushes to the seat and ankles, swelling the tubes there and pulling the non-stretch fabric taut. There is no connection to any machinery or computers in the plane. Libelle's relaxed protection is equivalent to a traditional suit (Relaxed GOR tol is 6.5, + 1.1G and Relaxed ROR tol is 5.2,+ 0.6G). The advantages of this suit are :



• Only one type of straining is required at higher G-levels i.e. only muscular straining and no respiratory straining .Respiratory straining is counter- productive at times.



• The pilots reports decreased fatigue and easier speech.



• Straining (muscular straining only) gives 9G protection.



• Cardiac output and venous return was good with the Libelle suit .



• When compared to COMBAT EDGE which uses PPB and gives 8.5G- GOR and 8.8G-ROR protection, the Libelle suit gives 9G.



• It greatly enhances the protective effectiveness of peripheral muscular straining.

The only drawback seems to be that the proper management of straining remains essential to avoid G-LOC occurring.

Sequentially inflating AGS :
In this suit the calf bladders are Siamese, but the thigh and abdominal bladders are separately controllable. The microprocessor software makes it possible to control the pressurization (through feedback loop) in each bladder, and to control phasing between bladder inflation. This uses individual solenoid valves, controlled from the input/output ports on the microprocessor, for bladder pressurization. This suit when used with the adaptive valve to provide periodic episodes of high pressure, results in a milking action during extended G-exposures. It therefore, materially assists venous return periodically to overcome the inevitable compromise of reduced venous return, observed in conventional suits.

The Indian AGS development has been keeping apace with the developemental technology of the aircrafts being used in the Indian Air Force. These suits have been designed by DEBEL (Defense Bioengineering and Electro-medical Laboratory), and are :

• AGS Mk I: This is a Polyester/cotton overall with a detachable bladderof neoprene coated nylon fabric and comes in 12 sizes



• AGS Mk II: This AGS is currently being used in the IAF.It is a cutaway AGS which provides for greater mobility and reduced heat loads. The cutaway is at the seat, crotch and knees. The outer fabric is heavy duty nylon fabric (high strength and low extensibility) and the bladders are made of natural rubber. There is provision of adjusting laces and zippers. It comes in 05 sizes and the protection offered is 1-1.2G



• AGS Mk III:This suit has been designed for aircrew operating in hot and humid conditions with the outer fabric is nylon/and inner cotton. The nylon provides the strength and cotton caters to the sweat absorption requirement. It has 4 pockets, slides fasteners, zippers, concealed adjustment lacing and a spring reinforced rubber hose which can take a variety of end connectors to fit various aircraft PEC. Basically designed for use in Harriers.



• AGS Mk IV:: Developed for use in high performance aircrafts like the Mirage, MiG29 and the LCA, it is very similar to the Mk II except that the bladders are large, and can withstand high pressures. Provisions for detachable thigh pockets, polyester zip and comfort zippers have been made.

PBG (Pressure Breathing for +Gz) The PBG systems being used in the world are in the USAF (F-15 and F-16), RAF (Hawk), the German Air Force, the Spanish Air Force, the French Air Force(Rafale), the Italian (Eurofighter 2000) and the Swedish (Grippen). These Air Forces have PBG- incorporation programmes at various stages of usage. Some of the more recent ones in use are discussed :-

• COMBAT EDGE: One of the lastest advances in acceleration protection is the Combined Advanced Technology Enhanced Designed G-Ensemble, COMBAT EDGE. Multiple studies have proven the effectiveness of enhancing +Gz tolerance with positive pressure breathing (PPB). PPB passively augments blood pressure and facilitates inspiration, thereby reducing the pilot's physiological workload and improving oxygenation. Historically, pulmonary overpressure leading to pneumothorax limited the operational usefulness of positive pressure breathing systems. However, COMBAT EDGE fielded between 1990 and 1992, combines PPB with a chest counter-pressure vest, to remove overpressure concerns. The standard G-suit, worn with Combat Edge, optimizes the upper body blood volume as it diminishes blood pooling to the legs and abdomen. The Combat Edge helmet has an inflating bladder at the back of the neck, which improves the mask seal. Positive pressure is then uniformly applied through a modified regulator to the oxygen mask and chest suit (jerkin). During passive inhalation, the positive pressure at the mask/mouth is directly transmitted into the lungs and chest cavity, and then indirectly to the heart and large arteries. The chest suit provides external chest counterpressure to increase pilot comfort and provide more efficient G protection. Positive pressure is scheduled and starts blending in at 4-5 G, reaching a maximum of 60 mmHg of pressure at 9 G. The USAF has employed the facility in its high performance aircraft, F-15 and F-16.



• The Tactical Life Support System(TLSS): Used in US Navy. Combinations of an improved Canadian torso garment and an extended coverage 5-bladder anti-G suit into a single integrated garment. Inflight SACM endurance was twice that of controls with AGSM wearing anti-G suit. In trials in both F-15 and F-16 of USAF, brought enthusiastic aircrew acceptance of PBG, but poor acceptance of the garment integration concept.



• The Northrop advance protection system: Similar to COMBAT EDGE, but incorporates larger coverage of the pilot's body, using pneumatic suits (legs, torso and arms) with pressure of 12-13 psi and PPB mask pressure of 60 mmHg. It is capable of providing altitude, acceleration, chemical and biological protection in one ensemble.



• Sustained Tolerance to Increased +Gz (STING), DCIEM, Canadian Air Force: This AGS consists of a 90% coverage anti-G suit, a chest counter pressure Jerkin, a Gentex helmet with an air to-tightening mask and a man-mounted PBG O2 regulator, which provides PPB in response to the inflation of the anti -G suit. A relaxed ROR tolerance of 7.5G, without AGSM has been reported.



• Enhanced +Gz-protection system in Hawk: Employs PBG with chest counter -pressure and full coverage anti -G trousers (93% of area below the umbilicus). Centrifuge evaluation with ROR profiles revealed a mean relaxed ROR +Gz-tolerance of 8.3G



• Rafale aircraft: PBG with counterpressure at 0.26 psi/G (13.4 mmHg/G) and initiated at 4G. AGS provided is the ECAGS (1psi/G begins at 2G). Assisted with 290 seat back angle, raised arms and legs



• Indian Air Force: The Su-30 has the facility of PBG (max pressure 600 mmWG). Mask to suit Pressurisation ratio is 1:3.2. In the Su-30, this facility is not being utilised presently, due to high heat loads of the flying clothing assembly. However, PBG indoctrination during the AFAIC uses 30 mmHg unassisted PBG during SACM runs, through a MK-20 regulator and oronasal mask. The initial response is encouraging. The LCA will have PBG capabilities (12mmHg/+Gz from 4G, max of 60 mmHg at 9G).

The advantage of PBG is that 1 to 4G protection is achieved easily. The method is a passive, unobtrusive and less fatiguing procedure. It makes AGSM easier and serves as an early warning for impending High +Gz for the rear pilot. Compatibility with HMDs/NBC is present It prolongs the SACM tolerance and has a high aircrew acceptability. However, expiration becomes active (Physiological reversal) and is difficult, a false sense of confidence is instilled and mobility and comfort are restricted. There is increased thermal stress. There is a need to increase the tension of the mask for adequate seal and overpressure can lead to pneumothorax and acceleration atelectasis.

PBG is a new and advanced countermeasure, but is not a panacea against G-LOC. PBG will not protect against rapid- onset high G. The new systems will provide improved +Gz tolerance and decrease fatigue, allowing aviators to accomplish the mission. The future lies in the use of combined methods for +Gz protection.

Anti-G Straining Manoeuvre (AGSM)
One of the most important methods for protection against the effect of 'G' is the Anti-G straining manoeuver (AGSM). This is a physiological method of protection and the pilot uses his own body to protect himself. It requires no additional equipment and can be combined with other forms of protection. Since the early days of aviation, muscular straining as a means of protection against the effects of headward acceleration (+Gz) had been recognised. Pilots noticed that straining and tensing of the muscles during acceleration, postponed greyout or blackout. Though various methods of AGSM are described, the most commonly used are M-I and L-I manoeuvers. The only difference between the L-1 and M-1, is in the position of the glottis, the trap door at the top of the airway. In M-I, straining (forceful exhalation) is done against a partially closed glottis. Some pilots prefer L-I manoeuver as it is less noisy and hence does not interfere as much with R/T communication, nor does it irritate the vocal cords. As both L-I and M-I give the same amount of protection, the choice of the manoeuver is the personal preference of the pilot.

The first part of these manoeuvers consists of a quick inhalation (gasp) followed by a more prolonged forceful, straining type of exhalation, lasting not less than 2 seconds and no longer than 3. The quick inhalation sucks venous blood into the chest; the prolonged and forceful straining keeps the blood pressure up, between heart and brain. This breathing pattern is only one important element, but it needs to be performed correctly. If inhalation is prolonged, pressure in the chest drops, BP drops and LOC can occur. If exhalation is too prolonged, venous return to the chest is impeded, which reduces the volume of blood pumped by the heart and also results in LOC. For that reason, the straining phase should only last no more than 2-3 seconds. The straining pattern of the correct breathing manoeuver is somewhat similar to that used in weight lifting.

The other part of the manoeuver involves tensing the large muscles of arms, legs thighs and abdomen. About 75% of the blood volume is pooled in large veins in these areas and this blood must be squeezed back into the chest before the heart can pump it up, to the brain. The entire maneuver, like any motor skill, requires practice. Like any motor exercise, it requires strength, stamina and recurrent exposure to stay in shape for pulling G. Moreover, like any feat requiring strength, stamina and endurance, the response can be fatiguing, which is often the limiting factor.

Another important aspect of the AGSM is the timing of its performance. It is known that even when a pilot correctly performs the coordinated straining tensing manoeuver, the BP response lags somewhat. For that reason, it is vital to anticipate high G and begin straining early i.e. "get a jump on G". Once behind the power curve; i.e. peak G is already reached, it may be too late to catch up. The only course then is to back off the Gs and do it quickly.

The gain in tolerance, contributed by AGSM ranges from 0.9 to 3.2 G. The protective benefits of the AGSM are only, as great as the effectiveness of its performance. AGSM can be practiced on the ground at 1G itself, but this will not be useful as the pilot has to perform it for protection only at higher G levels. Doing the AGSM under high G, is a totally different experience from doing it at 1G, as there are a lot of distractions under high G. Thus, to be effective, AGSM must be learnt and done correctly at high G and so repeated practice will be required. Since practicing the AGSM in the aircraft could be dangerous, it is best to practice the manoeuver in a human centrifuge. Moreover, in the centrifuge, a trained medical officer monitors the subject. The subject can also be trained to maintain the correct timing of the respiratory pattern. Such apnea (holding breath) is not possible for more time during high sustained +Gz (HSG) and pilots tend to relax their muscles after sometime. They are taught to breathe as in M-1, along with muscular contractions during HSG and are thus able to perform in a better fashion. Several Air Forces all over the world are training their pilots to perform AGSM under HSG in a centrifuge. It is the safest and most cost effective means of preventing loss of aircraft and aircrew due to G-LOC. In the IAF ,Air HQ has accepted the necessity of High G training in the centrifuge at IAM.

Physical Conditioning
Training should also include a physical conditioning programme to improve strength and stamina. Recent studies have shown that weight lifting, especially exercises designed to strengthen the muscles of abdomen, upper arms and upper legs can increase tolerance. As a result of an increased maximal voluntary strength, less muscular effort is required to produce a certain absolute force. Similarly, time to exhaustion for a given absolute muscle tension level is probably prolonged after training. Weight training conditions the muscles and helps develop breathing patterns which minimize fatigue during maneuvers and are therefore strongly recommended. Such a programme also develops large muscles groups in the body, which permit more effective blood flow during tensing. On the other hand, excessive aerobic training such as running more than 15 to 20 miles a week, is not beneficial and in fact may be detrimental to +Gz tolerance. Distance runners tend to have high responsiveness of the autonomic nervous system, which slows down the heart. Besides lowering heart rates, they tend to have lower resting BP. Although great for longevity, their BP may rise slowly under G. Also, the straining maneuver which normally stimulates the vagus nerve, may lower the heart rate so much that insufficient blood is pumped to the brain.

Studies indicate that the frequency of aerobic training e.g. jogging, playing squash/tennis etc. should be for 2-3 days in a week for adequate improvement of the cardiovascular system. It is recommended that exercises should be done at the desired level for 20-30 minutes, 3 days a week; alternating aerobic conditioning days with weight training days. Running distance should not exceed 4-5 Km per day. The best way to ensure that weight training is actually done on a regular basis is to set aside space in the Squadron for the equipment and schedule the workouts at specific times i.e., make it convenient. Towards this end, all Mirage 2000 squadrons in France are equipped with weight training gymnasia. The Royal Swedish Air force and the USAF have also built and equipped weight training gymnasia exclusively for fighter pilots at their Air Force bases. Another benefit of this programme is the overall increased feeling of well being that accompanies the increased muscular and aerobic fitness.

Reclined Seat
Reclining the seat improves G tolerance by reducing the effective aortic valve/eye column height. The improvement in G tolerance is roughly linear with reduction in effective column height (i.e. at 75o seat back angle, column height is reduced to one half and G tolerance is almost doubled). At high G in the reclined position, G tolerance becomes progressively limited by pain from contact with the seat, from chest compression and from difficulty in inhaling, due to the increased weight of the anterior chest wall. These symptoms limit this technique to about 14-15 G maximum. Although reclined seats can dramatically improve G tolerance, they are seldom used because of difficulty, providing full use of displays and controls while providing adequate outside vision.

The use of reclined seats to increase G tolerance has only been partially incorporated into an operational fighter, the F-16, which has an inclined seat of about 30 degrees. Hypothetically, reclined seats would decrease the vertical distance between the heart and the brain, thereby decreasing the required blood pressure to maintain brain perfusion. Most research suggests that there is no significant increase in G tolerance until the seat is inclined 45 degrees. More recent studies have suggested that increased G tolerance in the reclined F-16 seat is due to greater leg elevation and hip flexion. This body position decreases venous pooling in the legs and increases circulating volume and blood pressure.

Centrifuge Training
Human centrifuges are categorized in terms of generations as per their year of manufacture and capabilities. Table-I describes four generations of centrifuges, which have been developed and have been used at various stages. The first generation of centrifuges had a single degree of freedom (DOF), slow onset rate and a passive cab. Second generation centrifuges had some visual task and two degree of freedom, where as third generation can generate upto 6G/sec rate of onset and some flight simulation.

Table : Generations of Human Centrifuges

PERIOD	GENERATION
1935 - 1964	First Generation: Single degree-of-freedom (DOF), slow G onset, passive cab.
1964 - 1984	Second Generation: Multi-axis (2 degree of freedom), some visuals, some of them having gimbaled cabs
1984 - 1998	Third generation: 6 G/sec, some flight simulation, gimbaled (active) cab.
1998 - present	Fourth Generation: 10 – 15 G/sec, full flight simulation, gimbaled (active) cab. (active) cab.

Fourth generation Centrifuge : The current new centrifuges belong to the fourth generation. These have uniform and very high-G onset and offset rates of 10-15G/sec, have full flight simulation capability with availability of pre-defined profiles. The gondola is multi-gimbaled with a payload capacity of 500 kg and available motions are in roll and pitch axes. The seat is adjustable and has tilt back facilities. Displays are multifunction and provide a field of vision of 1200 in horizontal and 400 in vertical direction. The centrifuge has Head-up-display, Head-down-display and light bar for peripheral light loss detection. A close circuit TV monitoring with two-way communication is provided. Facilities for medical monitoring are available in the form of ECG, EMG, EEG, EOG, ENG, DOPPLER, Evoked Potential, NIBP, SaO2, Blood Flow, Rheography and Temperature. The gondola is fitted with both central and side stick control with programmable modes for control viz. pre, combat, manual open mode and pilot in the loop mode. Air conditioning and PBG facility are also available. Currently the fourth generation centrifuges are available in UK, Sweden, France, Singapore and Japan. Though the latter three have slightly lower onset and offset rates, they can be clubbed in this generation as they have the rest of the capabilities.

THE FUTURE
Revolutionary design and two-dimensional vectored thrust have introduced an era of Supermaneuverable/ Superagile (SM/SA) aircraft. The X-31, which is already in the flight test phase of development, is going to be role model for the future generation of fighter aircraft. As opposed to the present generation of Air Superiority Fighter (ASF) aircraft, which maneuver in roll, pitch and yaw, with primary airframe acceleration restricted to the Z-axis these SM/SA will have significant excursion and acceleration in the X and Y axes also. This will usher a revolution in understanding of acceleration physiology. The human operator of this machine will be exposed to rapid changes of significant level of accelerative stress in different axis. Considering the well-known role of the push-pull effect in reducing the tolerance of + Gz, a new look in the physiological changes associated with rapid changes in the direction of forces on the human body have to be thought off. A new criterion for tolerance limits will be the need of the hour and protective devices, which are in current use, will require suitable modification.

One final area of "G-LOC protection" is the development of auto recovery systems. The computers in the current generation aircraft can fly without pilot input. If systems can be developed to correctly identify that a pilot has lost consciousness, the aircraft's computers can be programmed to recover the aircraft to straight and level flight. Problems with this concept at present are the unequivocal identification of loss of consciousness (LOC), the provision of appropriate pilot override capability and pilot acceptance of the machine doing the flying. While the problem of G-LOC has not been entirely solved, the technology to solve many of the problems and the training to use the technology are at hand.

Equally important are some experimental protective techniques for increasing +Gz tolerance, currently under investigation. These methods include :

• Pulsating G suits, synchronized to the electrocardiogram. This technique would provide a pulse superimposed on the systolic pulse, producing a higher systolic pressure at head level.



• Optimization of physical fitness training procedures. This may allow a more forceful straining manoeuver with less fatigue.



• Drugs to increase head level blood pressure on a short-term basis



• Combined acceleration Flight Simulator: CAFS, with an arm radius of 250 ft, which will be capable of simulating impact and/ ejection accelerations along with other desirable features of fourth generation centrifuge, has been conceived. It would use electromagnetic levitation and propulsion to propel the gondola around a track. Although research in this field is in infancy, study on the gimbaled centrifuge has indicated that multi-axis sustained acceleration can either enhance or reduce G-tolerance depending on direction and duration of the exposure. Lateral acceleration in conjunction with vertical acceleration can enhance G-tolerance where as longitudinal acceleration in addition to vertical acceleration can reduce the tolerance.

CONCLUSION
The present and future generation fighter aircraft having capabilities of generating 10- 15 G/ sec onset and offset rates, HSG potential and SM capabilities require pilots with a matching acceleration tolerance. A pilot's tolerance in terms of 12 +Gz and above is feasible with the available technology and can be learnt and refined in a simulated environment. Research in achieving higher capabilities and multiple axis acceleration physiology requires a simulator with similar performance parameters and physiological monitoring facilities. Newer protective methods and recovery modalities in case of pilot incapacitation can be studied and researched on an advanced Human Centrifuge/ Dynamic Flight Simulator. These centrifuges provide a safe environment and "friendly" facilities to train pilots to withstand the rigor of HSG and multi-axis acceleration. They cost less than a modern fighter jet and are essential tools for a modern Air Force.

It must be remembered, and briefed to aircrew, that even the new and advanced countermeasures are not a panacea against G-induced Loss of Consciousness. It will not protect against rapid onset High-G forces. It is worthwhile recollecting that the first G-LOC with COMBAT EDGE occurred in 1994, secondary to a poor L-1 staining manoeuver. Additionally, if aircrew members fight the PBG system (passive inhalation, active exhalation), they will tire quickly, and not realize the full benefit of this form of protection. The new systems will, therefore, improve G tolerance and decrease fatigue, allowing aviators to accomplish the mission better.

REFERENCES :

• Albery W. Human Centrifuges: The Old and the New. Safe Journal - Vol 29 (2) - Summer/Fall 1999.



• Protection to +12Gz. Aviat Space Environ Med. VOL 72, No. 5. May 2000.



• AGARD-AG-322. ISBN 92-835-0596-4. High-G Physiological Protection Training. December 1990.



• Malik Harish, Kapur R: Centrifuge Training for Aircrew. Indian Journal Aerospace Medicine, 1991; 35 (2): 6-8.



• Glaister D. Physiological principles and operational consequences of the high-sustained G environment, AGARD short course and Royal Air Force Institute of Aviation Medicine, December 1992.



• Human centrifuge in aeromedical evaluation-Navathe et al. Ind. J. Aeros